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<div class="chapter">
<div class="titlepage"><div><div><h1 class="title">
<a name="topics"></a>Chapter 8. Topics</h1></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="topics.html#topics.calling_python_functions_and_met">Calling Python
      Functions and Methods</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="topics.html#topics.calling_python_functions_and_met.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="topics.html#topics.calling_python_functions_and_met.argument_handling">Argument
        Handling</a></span></dt>
<dt><span class="section"><a href="topics.html#topics.calling_python_functions_and_met.result_handling">Result
        Handling</a></span></dt>
<dt><span class="section"><a href="topics.html#topics.calling_python_functions_and_met.rationale">Rationale</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="topics/pickle_support.html">Pickle support</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="topics/pickle_support.html#topics.pickle_support.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="topics/pickle_support.html#topics.pickle_support.the_pickle_interface">The Pickle
        Interface</a></span></dt>
<dt><span class="section"><a href="topics/pickle_support.html#topics.pickle_support.example">Example</a></span></dt>
<dt><span class="section"><a href="topics/pickle_support.html#topics.pickle_support.pitfall_and_safety_guard">Pitfall
        and Safety Guard</a></span></dt>
<dt><span class="section"><a href="topics/pickle_support.html#topics.pickle_support.practical_advice">Practical Advice</a></span></dt>
<dt><span class="section"><a href="topics/pickle_support.html#topics.pickle_support.light_weight_alternative_pickle_">Light-weight
        alternative: pickle support implemented in Python</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="topics/indexing_support.html">Indexing support</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="topics/indexing_support.html#topics.indexing_support.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="topics/indexing_support.html#topics.indexing_support.the_indexing_interface">The
        Indexing Interface</a></span></dt>
<dt><span class="section"><a href="topics/indexing_support.html#topics.indexing_support.index_suite_sub_classes">index_suite
        sub-classes</a></span></dt>
<dt><span class="section"><a href="topics/indexing_support.html#topics.indexing_support.indexing_suite_class"><code class="computeroutput"><span class="identifier">indexing_suite</span></code> class</a></span></dt>
<dt><span class="section"><a href="topics/indexing_support.html#topics.indexing_support.class_vector_indexing_suite">class
        <code class="computeroutput"><span class="identifier">vector_indexing_suite</span></code></a></span></dt>
<dt><span class="section"><a href="topics/indexing_support.html#topics.indexing_support.class_map_indexing_suite">class
        <code class="computeroutput"><span class="identifier">map_indexing_suite</span></code></a></span></dt>
</dl></dd>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="topics.calling_python_functions_and_met"></a><a class="link" href="topics.html#topics.calling_python_functions_and_met" title="Calling Python Functions and Methods">Calling Python
      Functions and Methods</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="topics.html#topics.calling_python_functions_and_met.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="topics.html#topics.calling_python_functions_and_met.argument_handling">Argument
        Handling</a></span></dt>
<dt><span class="section"><a href="topics.html#topics.calling_python_functions_and_met.result_handling">Result
        Handling</a></span></dt>
<dt><span class="section"><a href="topics.html#topics.calling_python_functions_and_met.rationale">Rationale</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.calling_python_functions_and_met.introduction"></a><a class="link" href="topics.html#topics.calling_python_functions_and_met.introduction" title="Introduction">Introduction</a>
</h3></div></div></div>
<p>
          The simplest way to call a Python function from C++, given an <a class="link" href="object_wrappers/boost_python_object_hpp.html#object_wrappers.boost_python_object_hpp.class_object" title="Class object"><code class="computeroutput"><span class="identifier">object</span></code></a> instance f holding the
          function, is simply to invoke its function call operator.
        </p>
<pre class="programlisting"><span class="identifier">f</span><span class="special">(</span><span class="string">"tea"</span><span class="special">,</span> <span class="number">4</span><span class="special">,</span> <span class="number">2</span><span class="special">)</span> <span class="comment">// In Python: f('tea', 4, 2)</span></pre>
<p>
          And of course, a method of an <a class="link" href="object_wrappers/boost_python_object_hpp.html#object_wrappers.boost_python_object_hpp.class_object" title="Class object"><code class="computeroutput"><span class="identifier">object</span></code></a> instance <code class="computeroutput"><span class="identifier">x</span></code> can be invoked by using the function-call
          operator of the corresponding attribute:
        </p>
<pre class="programlisting"><span class="identifier">x</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"tea"</span><span class="special">)(</span><span class="number">4</span><span class="special">,</span> <span class="number">2</span><span class="special">);</span> <span class="comment">// In Python: x.tea(4, 2)</span></pre>
<p>
          If you don't have an <a class="link" href="object_wrappers/boost_python_object_hpp.html#object_wrappers.boost_python_object_hpp.class_object" title="Class object"><code class="computeroutput"><span class="identifier">object</span></code></a> instance, <code class="computeroutput"><span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Python</span></code> provides two families of function
          templates, <a class="link" href="function_invocation_and_creation/boost_python_call_hpp.html#function_invocation_and_creation.boost_python_call_hpp.function_call" title="Function call"><code class="computeroutput"><span class="identifier">call</span></code></a> and <a class="link" href="function_invocation_and_creation/boost_python_call_method_hpp.html#function_invocation_and_creation.boost_python_call_method_hpp.function_call_method" title="Function call_method"><code class="computeroutput"><span class="identifier">call_method</span></code></a>, for invoking Python
          functions and methods respectively on <code class="computeroutput"><span class="identifier">PyObject</span><span class="special">*</span></code>s. The interface for calling a Python function
          object (or any Python callable object) looks like:
        </p>
<pre class="programlisting"><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">ResultType</span><span class="special">&gt;(</span><span class="identifier">callable_object</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">...</span> <span class="identifier">aN</span><span class="special">);</span></pre>
<p>
          Calling a method of a Python object is similarly easy:
        </p>
<pre class="programlisting"><span class="identifier">call_method</span><span class="special">&lt;</span><span class="identifier">ResultType</span><span class="special">&gt;(</span><span class="identifier">self_object</span><span class="special">,</span> <span class="string">"method-name"</span><span class="special">,</span> <span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">...</span> <span class="identifier">aN</span><span class="special">);</span></pre>
<p>
          This comparitively low-level interface is the one you'll use when implementing
          C++ virtual functions that can be overridden in Python.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.calling_python_functions_and_met.argument_handling"></a><a class="link" href="topics.html#topics.calling_python_functions_and_met.argument_handling" title="Argument Handling">Argument
        Handling</a>
</h3></div></div></div>
<p>
          Arguments are converted to Python according to their type. By default,
          the arguments <code class="computeroutput"><span class="identifier">a1</span><span class="special">...</span><span class="identifier">aN</span></code> are copied into new Python objects,
          but this behavior can be overridden by the use of <a class="link" href="function_invocation_and_creation/boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.functions" title="Functions"><code class="computeroutput"><span class="identifier">ptr</span><span class="special">()</span></code></a>
          and <code class="computeroutput"><span class="identifier">ref</span><span class="special">()</span></code>:
        </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">X</span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span>
<span class="special">{</span>
   <span class="special">...</span>
<span class="special">};</span>

<span class="keyword">void</span> <span class="identifier">apply</span><span class="special">(</span><span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">callable</span><span class="special">,</span> <span class="identifier">X</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">)</span>
<span class="special">{</span>
   <span class="comment">// Invoke callable, passing a Python object which holds a reference to x</span>
   <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">call</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;(</span><span class="identifier">callable</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">x</span><span class="special">));</span>
<span class="special">}</span>
</pre>
<p>
          In the table below, x denotes the actual argument object and cv denotes
          an optional cv-qualification: "const", "volatile",
          or "const volatile".
        </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                  <p>
                    Argument Type
                  </p>
                </th>
<th>
                  <p>
                    Behavior
                  </p>
                </th>
</tr></thead>
<tbody>
<tr>
<td>
                  <p>
                    <code class="computeroutput"><span class="identifier">T</span> <span class="identifier">cv</span>
                    <span class="special">&amp;</span></code> <code class="computeroutput"><span class="identifier">T</span>
                    <span class="identifier">cv</span></code>
                  </p>
                </td>
<td>
                  <p>
                    The Python argument is created by the same means used for the
                    return value of a wrapped C++ function returning T. When T is
                    a class type, that normally means *x is copy-constructed into
                    the new Python object.
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    T*
                  </p>
                </td>
<td>
                  <p>
                    If x == 0, the Python argument will be None. Otherwise, the Python
                    argument is created by the same means used for the return value
                    of a wrapped C++ function returning T. When T is a class type,
                    that normally means *x is copy-constructed into the new Python
                    object.
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    boost::reference_wrapper&lt;T&gt;
                  </p>
                </td>
<td>
                  <p>
                    The Python argument contains a pointer to, rather than a copy
                    of, x.get(). Note: failure to ensure that no Python code holds
                    a reference to the resulting object beyond the lifetime of *x.get()
                    may result in a crash!
                  </p>
                </td>
</tr>
<tr>
<td>
                  <p>
                    pointer_wrapper&lt;T&gt;
                  </p>
                </td>
<td>
                  <p>
                    If x.get() == 0, the Python argument will be None. Otherwise,
                    the Python argument contains a pointer to, rather than a copy
                    of, *x.get(). Note: failure to ensure that no Python code holds
                    a reference to the resulting object beyond the lifetime of *x.get()
                    may result in a crash!
                  </p>
                </td>
</tr>
</tbody>
</table></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.calling_python_functions_and_met.result_handling"></a><a class="link" href="topics.html#topics.calling_python_functions_and_met.result_handling" title="Result Handling">Result
        Handling</a>
</h3></div></div></div>
<p>
          In general, <code class="computeroutput"><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">ResultType</span><span class="special">&gt;()</span></code>
          and call_method&lt;ResultType&gt;() return ResultType by exploiting all
          lvalue and rvalue from_python converters registered for ResultType and
          returning a copy of the result. However, when ResultType is a pointer or
          reference type, Boost.Python searches only for lvalue converters. To prevent
          dangling pointers and references, an exception will be thrown if the Python
          result object has only a single reference count.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.calling_python_functions_and_met.rationale"></a><a class="link" href="topics.html#topics.calling_python_functions_and_met.rationale" title="Rationale">Rationale</a>
</h3></div></div></div>
<p>
          In general, to get Python arguments corresponding to a1...aN, a new Python
          object must be created for each one; should the C++ object be copied into
          that Python object, or should the Python object simply hold a reference/pointer
          to the C++ object? In general, the latter approach is unsafe, since the
          called function may store a reference to the Python object somewhere. If
          the Python object is used after the C++ object is destroyed, we'll crash
          Python.
        </p>
<p>
          In keeping with the philosophy that users on the Python side shouldn't
          have to worry about crashing the interpreter, the default behavior is to
          copy the C++ object, and to allow a non-copying behavior only if the user
          writes boost::ref(a1) instead of a1 directly. At least this way, the user
          doesn't get dangerous behavior "by accident". It's also worth
          noting that the non-copying ("by-reference") behavior is in general
          only available for class types, and will fail at runtime with a Python
          exception if used otherwise[1].
        </p>
<p>
          However, pointer types present a problem: one approach is to refuse to
          compile if any aN has pointer type: after all, a user can always pass *aN
          to pass "by-value" or ref(*aN) to indicate a pass-by-reference
          behavior. However, this creates a problem for the expected null pointer
          to None conversion: it's illegal to dereference a null pointer value.
        </p>
<p>
          The compromise I've settled on is this:
        </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
              The default behavior is pass-by-value. If you pass a non-null pointer,
              the pointee is copied into a new Python object; otherwise the corresponding
              Python argument will be None.
            </li>
<li class="listitem">
              if you want by-reference behavior, use ptr(aN) if aN is a pointer and
              ref(aN) otherwise. If a null pointer is passed to ptr(aN), the corresponding
              Python argument will be None.
            </li>
</ol></div>
<p>
          As for results, we have a similar problem: if ResultType is allowed to
          be a pointer or reference type, the lifetime of the object it refers to
          is probably being managed by a Python object. When that Python object is
          destroyed, our pointer dangles. The problem is particularly bad when the
          ResultType is char const* - the corresponding Python String object is typically
          uniquely-referenced, meaning that the pointer dangles as soon as call&lt;char
          const*&gt;(...) returns.
        </p>
<p>
          The old Boost.Python v1 deals with this issue by refusing to compile any
          uses of call&lt;char const*&gt;(), but this goes both too far and not far
          enough. It goes too far because there are cases where the owning Python
          string object survives beyond the call (just for instance, when it's the
          name of a Python class), and it goes not far enough because we might just
          as well have the same problem with a returned pointer or reference of any
          other type.
        </p>
<p>
          In Boost.Python this is dealt with by:
        </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
              lifting the compile-time restriction on <code class="computeroutput"><span class="keyword">char</span>
              <span class="keyword">const</span> <span class="special">*</span></code>
              callback returns
            </li>
<li class="listitem">
              detecting the case when the reference count on the result Python object
              is 1 and throwing an exception inside of <code class="computeroutput"><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;(...)</span></code> when <code class="computeroutput"><span class="identifier">U</span></code>
              is a pointer or reference type.
            </li>
</ol></div>
<p>
          This should be acceptably safe because users have to explicitly specify
          a pointer/reference for <code class="computeroutput"><span class="identifier">U</span></code>
          in <code class="computeroutput"><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span></code>,
          and they will be protected against dangles at runtime, at least long enough
          to get out of the <code class="computeroutput"><span class="identifier">call</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;(...)</span></code> invocation.
        </p>
</div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2015 David Abrahams, Stefan Seefeld<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>
      </p>
</div></td>
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